Manufacture of optical fiber performs, the glass blanks from which optical fibers are drawn, typically involves a rotating lathe, where pure glass or glass soot is formed on a rotating member by chemical vapor deposition or a modification thereof. All successful methods of fiber manufacture should assure that the optical quality and purity of the preform glass is high. In particular, the glass making up the central portion or core of the preform should be of the highest purity since most of the optical power in the fiber will be carried within this region. A significant advance in this direction occurred with the introduction of the so-called Modified Chemical Vapor Deposition (MCVD) process in which the glass-forming precursors are introduced into a rotating hollow starting tube, and glass material is deposited on the inside wall of the hollow tube. The better control over the reaction environment provided by this inside deposition process, allowed exceptionally pure glass to be produced in the critical core region.
The MCVD technique has evolved to a highly sophisticated manufacturing technique, and is widely used in commercial practice today. However, limiting aspects in MCVD and similar inside deposition processes are the size and quality of the starting tube and the total amount of glass that can be deposited carried out in an overcladding process. The overcladding process in general is inside a starting tube. The limitation on the total amount of deposited glass necessarily limits the number of distinct doped regions or segments of a given size that can be accommodated in a preform of this type.
Another preform fabrication technique, Vapor Axial Deposition (VAD), was developed in which the CVD-formed silica soot deposits and grows axially from a starting mandrel. In a subsequent manufacturing stage or stages, the soot body is purified, dried and sintered into pure glass. At some point, the mandrel is separated from the deposited body and the entire preform, unlike MCVD, may thus be made of CVD-deposited material. As a general proposition, VAD to methods are effective and widely practiced, but they still do not match the ability of MCVD to control precisely the radial deposition profile of index changing dopants such as germanium and fluorine. Because of this, VAD methods and other soot deposition/subsequent sintering methods such as Outside Vapor Deposition (OVD) are limited in the complexity of the fiber designs that can be efficiently produced.
Considering that in a single mode optical fiber the core and inner cladding together carry greater than 95% of the optical power but typically comprise less than 5% of the fiber mass, all manufacturing processes focus special attention on the fabrication of this region. This has resulted in approaches to preform manufacture, where the core and inner cladding regions of the preform are produced by a relatively advanced and expensive method, while the outer cladding, the bulk of the preform, may be produced by a less demanding, and less expensive process. The integration of the core rod and the cladding is carried out in an overcladding process. The overcladding process in general is described for example in U.S. Pat. No. 6,105,396 (Glodis et al), and PCT/EPT00/02651 (25 Mar. 2000), which are incorporated herein by reference for details of the general techniques. The overcladding process may consist of multiple steps, each adding a distinct cladding region, if this is required by the complexity of the desired fiber refractive index profile. The most prevalent process of this type is the so-called rod-in-tube method, where the core rod is made by a very high quality dopant-versatile process, and the cladding tube is often made of less expensive, lower purity or single composition glass. In the to rod-in-tube overcladding process, the core rod is inserted into the cladding tube, and the tube collapsed around the rod to form a unitary body. Again, multiple overcladding steps may be used and in some cases one or more of the final overcladding processes may be combined with the fiber drawing operation.
State of the art manufacture for very large preforms now makes use of core rods produced by Outside Vapor Deposition or Vapor Axial Deposition. If a tube overcladding process is used, suitable cladding tubes may be produced by soot deposition or extrusion of fused quartz. Making these very large cladding bodies with a soot based synthetic glass process leads to high quality glass but requires extensive processing and is relatively expensive. Large bodies of fused quartz are less expensive but are generally not of sufficient purity for large preforms. A more economical approach for making high quality cladding tubes is to use sol-gel techniques. This well-known procedure is described, for example, in J. Zarzycki, “The Gel-Glass Process”, pp. 203-31 in Glass: Current Issues, A. F. Wright and J. Dupois, eds., Martinus Nijoff, Boston, Mass. (1985). Sol-gel techniques are regarded as potentially less costly than other known preform fabrication procedures. Options for producing the cladding tubes are addressed here for completeness, but the focus of this invention is on the core rod. The term core rod is used for convenience since the core rod always contains the central core material. However, the rod may comprise inner cladding, or both inner and outer cladding, as well as the central core. These options will be described in more detail below.
For producing very high quality central core and inner cladding material, to the MCVD process would appear ideal. However, the MCVD starting tube can be a limiting factor in several ways. The most direct limitation is when the glass in the MCVD starting tube is simply not of sufficient quality and low loss for large state of the art preforms (where some fraction of the optical power would be carried by the starting tube material). If the initial tube quality limitation is avoided by the use of ultra pure (and typically expensive) material to fabricate the starting tube, the exposure of the tube to the oxy-hydrogen torch typically used in MCVD as a heat source can compromise the effective starting tube quality by the addition of hydroxyl ions to a significant depth. Finally, the desired refractive index profile may require a dopant level in the region provided by the starting tube glass that is not compatible with successful MCVD processing (viscosity, tube stability or heat transfer considerations).
It should be evident from the discussion above that the production of very large core rods for rod in tube methods appears to be most suitably accomplished by VAD or OVD type methods. While the MCVD process is capable, along with the VAD and OVD processes, of producing very high quality glass, the MCVD glass is deposited inside a starting tube which, because of the reasons outlined above, can disadvantageously limit the application of the rod in tube method to preforms below a given size.